Buffer layer structures for stabilization of a lithium niobate device

ABSTRACT

An optical waveguide device including an electro-optical crystal substrate having a top surface and a bottom surface; an optical waveguide path formed within a surface of the electro-optical crystal substrate; at least one electrode positioned above the optical waveguide path for applying an electric field to the optical waveguide path; and a silicon titanium oxynitride layer and a connecting layer for interconnecting the silicon titanium oxynitride layer to another surface of the electro-optical crystal substrate that is opposite to the surface in which the optical waveguide path is formed.

This is a divisional application of pending prior application No.10/143,885, filed on May 14, 2002, now U.S. Pat. No. 6,661,934, which isa Continuation-in-Part of prior application No. 10/035,193, filed onJan. 4, 2002, now U.S. Pat. No. 6,654,512.

BACKGROUND OF THE INVENTION

1. Field of theInvention

The present invention relates to an optical waveguide modulator, andmore particularly, to the provision of improved thermal and temporalbias stability in optical waveguide devices.

2. Discussion of the Related Art

Mach Zehnder interferometers (MZI's) used as optical modulators are ofgreat interest for high data rate fiber optical communications systems.A great deal of research has been carried out to develop this type ofdevice since its introduction in the mid-70's. The practicality ofTi-diffused LiNbO₃ waveguide systems has allowed wide introduction ofthese devices in current optical communication systems.

FIG. 1 illustrates a plan view of a related art Z-cut lithium niobateMach-Zehnder interferometer used for modulation of an optical signal. Anoptical waveguide path 4 is formed inside a surface of a lithium niobate(LiNbO₃) substrate 1 that splits into a first path 4 a and a second path4 b and then recombines back into a single path 4′. The opticalwaveguide paths 4 a and 4 b may be formed by diffusion of a metal, forexample titanium, or with other dopants that will form an optical pathin the lithium niobate substrate 1. An electric field is applied to thefirst optical waveguide path 4 a and the second optical waveguide path 4b via electrodes 2 a and 3, respectively, that are positioned over thefirst and second optical waveguide paths. Specifically, the electrode 2a over the first optical waveguide path 4 a is a ground electrode andthe electrode 3 over the second optical waveguide path 4 b is an inputelectrode. In addition, another ground electrode 2 b is positioned onthe substrate so that ground electrodes 2 a and 2 b are on each side ofthe input electrode 3 for further control of the electric fields appliedto the first and second optical waveguide paths 4 a and 4 b. Theelectrodes 2 a, 2 b and 3 are separated from the substrate 1 by a bufferlayer 5. The application of the electric field changes the refractiveindex of an optical waveguide path in proportion to the amount ofelectric field applied. By controlling the amount of electric fieldapplied via the electrodes 2 a, 2 b and 3, an optical signal passingthrough the optical waveguide paths can be modulated.

FIG. 2 is cross-sectional view of the related art Z-cut lithium niobateMach-Zehnder interferometer along line B-B′ in FIG. 1. The buffer layer5 is comprised of a transparent dielectric film and is positionedbetween the electro-optical crystal substrate 1 and the electrodes 2 a,2 b and 3. The buffer layer 5 prevents optical absorption of the opticalmode by the metal electrodes 2 a and 3. However, the buffer layer 5allows electric fields that emanate from the electrode 3 to affect arefractive index change in either or both the first optical waveguidepath 4 a or the second optical waveguide path 4 b. Typically, silicondioxide (SiO₂) is used as the buffer layer due to its opticaltransparency at 1.55 microns and its low dielectric constant.

FIG. 2 also illustrates that electro-optical crystal substrate 1 of therelated art Z-cut lithium niobate Mach-Zehnder interferometer is formedso that a Y axis of the crystal orientation extends in a longitudinaldirection of the lithium niobate substrate 1 along the waveguide paths 4a and 4 b. The Z axis of the crystal orientation extends in thedirection of the thickness of the electro-optical crystal substrate 1such that the top and bottom surfaces of the lithium niobate substrate 1are respectively −Z and +Z faces in terms of the crystal latticestructure of the substrate. The optical waveguide paths are commonlydenoted as being within the −Z face of the lithium niobate substrate.

One of the practical difficulties in the early introduction of Z-cutLiNbO₃ devices was the pyroelectric sensitivity of LiNbO₃, whichresulted in the development of large internal fields within the deviceswhen subjected to temperature changes or gradients across the device.This is because a change in temperature causes a change in thespontaneous polarization due to the ferroelectric properties of LiNbO₃.As illustrated in FIG. 2, this results in an imbalance of charge betweenthe Z faces of the electro-optical crystal substrate 1, so that anelectric field is generated in the Z direction perpendicularly along thewaveguide paths 4 a and 4 b of the device. Due to the very highresistivity of LiNbO₃, these charges take a long time to travel throughthe electro-optical crystal substrate 1 and neutralize themselves. Thisimbalance of charge impedes or lessens the effect of the electricalfields from the electrodes 2 a, 2 b and 3 on the waveguide paths 4 a or4 b, thus decreasing the effectiveness or control in modulating opticalsignals. Early modulators were highly susceptible to thermal changes andstrict environmental controls were necessary for thermal stabilizationof the devices.

An early approach to maintain or prevent loss of modulation control dueto thermal effects was to bleed off or counteract the imbalance ofcharge between the Z faces of a LiNbO₃ substrate. C. H. Bulmer et al.(one of the authors is an inventor in this application), “PyroelectricEffects in LiNbO₃ Channel Waveguide Devices,” Applied Physics Letters48, p. 1036, 1986 disclosed that metallizing the Z faces, andelectrically connecting them with a high conductivity path to allow theunbalanced charge to neutralize rapidly, resulted in improved thermalstability of an X-cut device. Nonetheless, in Z-cut devices, thisapproach is difficult since the waveguide paths are on the Z face, and ametalized layer on this face would short out the electrodes of thedevice, making the device ineffective or inoperable.

Instead of a metallization layer, P. Skeath et al. (one of the authorsis an inventor in this application), “Novel Electrostatic Mechanism inthe Thermal Stability of Z-Cut LiNbO₃ Interferometers,” Applied PhysicsLetters 49, p 1221, 1986 and I. Sawaki et al., Conference on Lasers andElectro-Optics, MF2, PP. 46-47, San Francisco, 1986 suggested asemiconducting or semi-insulating layer on the Z face under theelectrodes of a Z-cut device. The semiconducting or semi-insulatinglayer would transfer the unbalanced charge between the Z faces of theLiNbO₃ substrate but not short out the electrodes. Although X-cutdevices are commonly treated by providing metal layers or otherconductive layers on the Z faces and interconnecting the conductivelayers, research continues as to what semiconductor or semi-insulatinglayer can be best or appropriately specified for use with Z-cut devices.

Approaches attempted in the past have included Indium Tin Oxide (ITO),Silicon (Si), and Silicon Titanium Nitride (Si_(x)Ti_(y)N_(z)) layers,which are applied in place of or above the usual SiO₂ buffer layer on aZ-cut optical waveguide device. Minford et al., “Apparatus and Methodfor Dissipating Charge from Lithium Niobate Devices, U.S. Pat. No.5,949,944, Sep. 7, 1999, which is hereby incorporated by reference,proposes a silicon titanium nitride layer that has the advantage ofadjustable resistivity by adjustment of the silicon/titanium ratio.However, control of the resistivity is unsatisfactory due to oxygencontamination in the silicon titanium nitride buffer layer, whichresults from residual background gases in the deposition system. Thisresults in unacceptable run-to-run variation in the resistivity of asilicon titanium nitride buffer layer. Furthermore, the depositionsystem for a silicon titanium nitride buffer layer includes a sputteringprocess that requires a variety of targets with varying compositions tovary the composition of the buffer layer over a desired range and thus,is not a practical process with suitable control of the resistivity.

The effect of the electric field and the consistency of the effect ofthe electric field over time (i.e. temporal stability) applied to thewaveguide paths are greatly affected by characteristics of the bufferlayer. The amount of electric field from the electrodes that is affectedby charge variations within the buffer layer or by the charge imbalancein the lithium niobate substrate is referred to as the bias drift of adevice. Temporal stability of Z-cut Ti diffused LiNbO₃ devices has beendiscussed in Seino et al., “Optical Waveguide Device,” U.S. Pat. No.5,404,412, Apr. 4, 1995, which is hereby incorporated by reference.Specifically, Seino et al. shows that repeated or constantly appliedvoltages across a buffer layer in a Z-cut device will ultimately resultin a buffer layer developing charge screening processes thatsignificantly reduces the electric field across the waveguide paths.Seino et al. further shows that by adding titanium and indium oxides toan SiO₂ buffer layer, the resulting bias drift was reduced (delayed intime).

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to LiNbO₃ devices thatsubstantially obviates one or more of the problems due to limitationsand disadvantages of the related art.

One aspect of the invention relates to pyroelectric or thermalstabilization of LiNbO₃ electro-optical devices.

Another aspect of the invention relates to temporal stabilization ofLiNbO₃ electro-optical devices.

Also, another aspect of the invention relates to a process enablingcontrol of resistivity in a buffer layer structure for electro-opticaldevices.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a plan view of a related art Z-cut lithium niobateMach-Zehnder interferometer;

FIG. 2 is cross-sectional view of the related art Z-cut lithium niobateMach-Zehnder interferometer along line B-B′ in FIG. 1;

FIG. 3 is a chart of the resistivity vs. oxygen concentration for anexemplary buffer layer of the present invention;

FIG. 4 is a cross-sectional view of a Z-cut lithium niobate waveguidedevice according to a first exemplary embodiment of the presentinvention;

FIG. 5 is a chart of the thermal sensitivity for a Z-cut lithium niobatewaveguide device according to the first exemplary embodiment of thepresent invention;

FIG. 6 is a cross-sectional view of a Z-cut lithium niobate waveguidedevice according to the second exemplary embodiment of the presentinvention;

FIG. 7 is a cross-sectional view of a Z-cut lithium niobate waveguidedevice according to a third exemplary embodiment of the resentinvention;

FIG. 8 is a cross-sectional view of a Z-cut lithium niobate waveguidedevice according to a fourth exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention relate to a silicontitanium oxynitride buffer layer structure formed by O₂ beingdeliberately introduced during a sputtering deposition process with asilicon nitride titanium nitride target in a nitrogen and a non-reactivegas environment. The O₂ concentration during the deposition of thesilicon titanium oxynitride is a tuning parameter for adjusting theresistivity of a resulting buffer layer. In the alternative or inaddition to adjusting the O₂ concentration, the N₂ concentration may bevaried during deposition as another tuning parameter for adjusting theresistivity of the resulting buffer layer. Due to the ease incontrolling the process, buffer layers over a broad range ofresistivities can be produced from a single fixed ratio siliconnitride-titanium nitride target. Furthermore, a buffer layer having aresistivity gradient can be produced by controlling the amount of O₂and/or N₂ during the deposition of the buffer layer.

In accordance with the present invention, a variety of exemplary silicontitanium oxynitride films were fabricated by sputtering from a 50%Si₃N₄-50% TiN target in an atmosphere containing a non-reactivesputtering gas (i.e., Ar), a predetermined amount of N₂, and apredetermined amount of O₂. Using the 50% Si₃N₄/50% TiN target and twochamber pressures of 1 and 5 m torr, gas flows of 40 sccm (standardcubic cm) Ar and 20 sccm N₂ were introduced into the reaction chamberfor each film. The resistivity of each film was controlled by a flowrate of a 95% Ar/5% O₂ mixture that was between and 0.5-10 sccm for eachfilm. Although exemplary conditions have been set forth above, themixture of gases and flow rates may be varied or otherwise different forSi₃N₄/TiN targets having different percentages of compositions.

After deposition of the films, the resistivities were measured and thefilm oxygen content, in atomic percent, was measured using XPS analysis.The results are shown in the chart of FIG. 3, where resistivity vs.oxygen concentration is plotted. The film resistivities varied fromapproximately 10⁵ to 10¹⁰ Ohm-cm while oxygen concentrations in thefilms varied from approximately 5% to 65% (atomic %). The resistivitiesof the films were stable over periods of months, and were also stablewhen subjected to 125 degrees C heat treatment in air overnight. SinceO₂ and N₂ gas flow in the chamber was precisely controlled, and the O₂chamber concentration was large compared to any residual backgroundgases, run-to-run reproducibility of the resistivity was good.

In continued accordance with the present invention, the use of otherpercentage compositions for Si₃N₄ and TiN as the Si₃N₄-TiN target willfurther extend the range of resistivities that can be achieved by theprocess above. For example, a higher percentage of TiN (i.e., 40TSi₃N₄/60% TiN) in the target will allow for a range of lowerresistivities that can be accurately obtained redundantly and a higherpercentage of Si₃N₄ (i.e., 60% Si₃N₄/40% TiN) will allow for a range ofhigher resistivities that can be accurately obtained redundantly. For apredetermined fixed percentage composition Si₃N₄-TiN target, apredetermined resistivity, within the range of resistivities for thatfixed percentage ratio target, can be accurately obtained redundantly bycontrol of the O₂ concentration or N₂/O₂ ratio in the sputtering gas.

FIG. 4 is a cross-sectional view of a first exemplary embodiment of thepresent invention having a silicon titanium oxynitride film as a bufferlayer for suppressing thermal (i.e., pyroelectric) effects in abroadband traveling wave Mach-Zehnder interferometer. The Mach-Zehnderinterferometer includes a Z-cut lithium niobate substrate 10 in whichoptical waveguide paths 14 a and 14 b are formed by diffusion of adopant, for example titanium, to form the optical waveguide paths.Electrodes 12 a and 13 are respectively positioned above (i.e.vertically separated from) or directly above (i.e. vertically separatedfrom and overlapping) the first optical waveguide path 14 a and thesecond optical waveguide path 14 b. Specifically, the electrode 12 aover the first optical waveguide path 14 a is a ground electrode and theelectrode 13 over the second optical waveguide path 14 b is an inputelectrode. In addition, another ground electrode 12 b is positioned onthe substrate so that ground electrodes 12 a and 12 b are on each sideof the input electrode 13 for further control of the electric fieldsapplied to the first and second optical waveguide paths 14 a and 14 b.An undoped SiO₂ buffer layer 15 is formed on the surface of the Z-cutlithium niobate substrate 10 above the optical waveguide paths 14 a and14 b. A silicon titanium oxynitride buffer layer 16 is positionedbetween the undoped SiO₂ buffer layer 15 and the electrodes 12 a, 12 band 13, and above the optical waveguide paths 14 a and 14 b in the Z-cutlithium niobate substrate 10. The thickness of the silicon titaniumoxynitride buffer layer 16 is about 0.1-1.0 micron and the thickness ofthe SiO₂ buffer layer 15 is about 0.1-1.5 micron. The undoped SiO₂buffer 15 layer isolates the optical fields within the waveguide paths14 a and 14 b from the metal electrodes with a low dielectric constantbut yet has an optical transparency for an optical wavelength of 1.55microns.

A connecting layer 17 interconnects the silicon titanium oxynitridebuffer layer 16 above the −Z face (top surface) and the +Z face (bottomsurface) of the lithium niobate substrate 10 on the sides (as shown inFIG. 4) or on one side of the lithium niobate substrate 10. A carbon orsilver paint, solder paste, conductive epoxy or other conductivematerials can be used as the connecting layer 17. Furthermore, resistivematerials such as semiconductors or ceramics may be used as theconnecting layer 17. In the alternative, the silicon titanium oxynitridebuffer layer 16 and the +Z face (bottom surface) of the lithium niobatesubstrate 10 can both be commonly connected to the housing (not shown)of the device. Although the connecting layer 17 is shown in FIG. 4 asoverlapping the top surface of the silicon titanium oxynitride 10,alternatively, the connecting layer 17 can just contact a side surfaceof silicon titanium oxynitride buffer layer 16.

Three devices of the first exemplary embodiment, as described above,were manufactured having different silicon titanium oxynitride bufferlayer resistivities that varied from approximately 8.0×10⁴−2.5×10⁵Ohm-cm. The resistivity of the SiO₂ buffer layer 15 under the silicontitanium oxynitride buffer layer 16 was approximately 2×10¹¹ Ohm-cm. Thethree devices were heated on a hot plate and the intrinsicinterferometer phase change was monitored as the temperature of thedevice was increased from 25 to 45 degrees C and then decreased from 45to 25 degrees C. The intrinsic phase of an interferometer will change asthe temperature changes due to thermal (i.e., pyroelectric) effectswithin the lithium niobate substrate 10. The intrinsic phase of theinterferometers due to thermal effects for the three devices with asilicon titanium oxynitride buffer layer and for a control device withjust a SiO₂ buffer layer are shown in FIG. 5. The resistivity of theSiO₂ buffer layer on the control sample was approximately 2×10¹¹ Ohm-cm.Specifically, FIG. 5 shows the changes in intrinsic phase of theinterferometers as the temperature is increased (from left to right) andthen as the temperature is decreased (from right to left). The threedevices with a silicon titanium oxynitride buffer layer show thermalsensitivities (change in interferometer phase/change in temperature) ofapproximately 2-2.5 degrees/degree C, nearly an order of magnitudesmaller than the control sample without the silicon titanium oxynitridebuffer layer. Therefore, the chart in FIG. 5 demonstrates that a silicontitanium oxynitride buffer layer achieves thermal stabilization andmitigates pyroelectric effects. In addition, the three devices did notexhibit any other performance degradation due to the presence of thesilicon titanium oxynitride buffer layer.

A silicon titanium oxynitride buffer layer is useful for temporal aswell as thermal stabilization of Z-cut LiNbO₃ devices. As discussedabove with regard to FIG. 4, the SiO₂ buffer layer on a lithium niobatesubstrate isolates the optical field of the waveguide paths from themetal electrodes. Furthermore, SiO₂ has a refractive index ofapproximately 1.45 that is significantly less than the lithium niobateand thus prevents that optical signal from being absorbed by theelectrodes. However, a silicon titanium oxynitride layer with anappropriate compositional structure can replace a SiO₂ buffer layer.Therefore, a buffer layer structure can be formed in a single layer in asingle process.

For this application, the desired film properties for a single bufferlayer structure of silicon titanium oxynitride are different from theSiO₂/silicon titanium oxynitride buffer layer structure described abovewith regard to FIG. 4 that was used only for thermal stabilization of anoptical waveguide device. For both thermal and temporal stabilization, asilicon titanium oxynitride film should have a low N/O ratio at theinterface between a buffer layer and lithium niobate substrate 10 tosomewhat mimic the performance of a SiO₂ buffer layer, but yet havecontrolled degrees of resistivity throughout the layer. This can beachieved by appropriate control of the N₂/O₂ ratio in the atmosphere ofthe sputtering deposition process when the silicon titanium oxynitridebuffer layer is deposited on the lithium niobate substrate. Increasingthe N₂/O₂ ratio during deposition causes a gradient in the N/O ratiowithin the layer, and thus forms a graded silicon titanium oxynitridebuffer layer. A lower N₂/O₂ ratio during the initial part of thedeposition creates a lower N/O ratio within the graded silicon titaniumoxynitride buffer layer adjacent to the interface between the bufferlayer and lithium niobate substrate. The lower N/O ratio in the lowerpart of the buffer layer will serve to maintain optical confinement andimprove temporal stabilization. A higher N₂/O₂ ratio during the latterpart of the deposition creates a higher N/O ratio within the gradedsilicon titanium oxynitride buffer layer at the top surface of thebuffer layer. The high N/O ratio in the upper part of the buffer layerimproves thermal stabilization.

FIG. 6 is a cross-sectional view of a second exemplary embodiment of thepresent invention having a silicon titanium oxynitride film as a singlebuffer layer for suppressing both thermal and temporal effects in abroadband traveling wave Mach-Zehnder interferometer. The Mach-Zehnderinterferometer includes a Z-cut lithium niobate substrate 20 in whichoptical waveguide paths 24 a and 24 b are formed by diffusion of adopant, for example titanium, to form the optical waveguide paths.Electrodes 22 a and 23 are respectively positioned above or directlyabove the optical waveguide paths. In addition, another electrode 22 bis positioned on the substrate so that electrodes 22 a and 22 b are oneach side of the electrode 23, which is an input signal electrode. Agraded silicon titanium oxynitride buffer layer 26 having gradients inthe Si/Ti ratio and/or the N/O ratio is positioned on (i.e. in contactwith) the Z-cut lithium niobate substrate below (i.e. verticallyseparated from) the electrodes 22 a, 22 b and 23, and above the opticalwaveguide paths 24 a and 24 b in the Z-cut lithium niobate substrate 20.The thickness of the graded silicon titanium oxynitride buffer layer 26is about 0.1-1.5 micron. Similarly as previously discussed with regardto FIG. 4, a connecting layer 27 on the sides of the lithium niobatesubstrate 20 interconnects the graded silicon titanium oxynitride bufferlayer 26 on the −Z face of the lithium niobate substrate 20 to the +Zface of the lithium niobate substrate 20.

The graded silicon titanium oxynitride buffer layer 26 may be doped withindium, or other rare earth metals, in metal or oxide form to furtherimprove temporal stability. Preferably, the rare earth metals should atleast be within the graded silicon titanium oxynitride buffer layer 26adjacent the interface between the buffer layer and lithium niobatesubstrate 20. This can be accomplished by exposing another target (i.e.,multi-source deposition) within the deposition chamber during theinitial deposition of the graded silicon titanium oxynitride bufferlayer 26.

By increasing the ratio of N₂ to O₂ during the sputtering deposition ofthe graded silicon titanium oxynitride buffer layer, the N/O ratiowithin the layer maintains optical confinement and improves both thermaland temporal stabilization of the device. However, the invention asdescribed above with regard to FIG. 6 can also be applied as a two layerbuffer structure instead of a single layer structure having a gradient.The two layer buffer structure further increases the range ofresistivities available for each layer since a different percentagefixed target can be used for the deposition of each layer.

FIG. 7 is a cross-sectional view of the third exemplary embodiment ofthe present invention of two buffer layers of silicon titaniumoxynitride for suppressing both thermal and temporal effects in abroadband traveling wave Mach-Zehnder interferometer. The Mach-Zehnderincludes a Z-cut lithium niobate substrate 30 in which optical waveguidepaths 34 a and 34 b are formed by diffusion with a dopant, for exampletitanium, to form the optical waveguide paths. Electrodes 32 a and 33are respectively positioned above or directly above the opticalwaveguide paths. In addition, another electrode 32 b is positioned onthe substrate so that electrodes 32 a and 32 b are on each side of theelectrode 33, which is an input signal electrode. A first silicontitanium oxynitride buffer layer 36 having a first Si/Ti ratio and afirst N/O ratio is positioned on the Z-cut lithium niobate substrate 30below electrodes 32 a, 32 b and 33, and above the optical waveguidepaths 34 a and 34 b. A second silicon titanium oxynitride buffer layer37 having a second Si/Ti ratio and a second N/O ratio is positioned onthe first silicon titanium oxynitride buffer layer below electrodes 32a, 32 b and 33, and above the optical waveguide paths 34 a and 34 b. Thethickness of the first silicon titanium oxynitride buffer layer 36 isabout 0.1-1.5 micron and the thickness of the second titanium oxynitridebuffer layer 37 is about 0.1-1.5 micron. Similarly as previouslydiscussed with regard to FIG. 4, a connecting layer 38 on the sides ofthe lithium niobate substrate 30 interconnects the second silicontitanium oxynitride buffer layer to the +Z face (bottom surface) of thelithium niobate substrate 30.

The first Si/Ti ratio in the first silicon titanium oxynitride bufferlayer 36 is larger than the second Si/Ti ratio in the second silicontitanium oxynitride buffer layer 37 for temporal stabilization of theMach-Zehnder device. The second N/O ratio in the second silicon titaniumoxynitride buffer layer 37 is larger than the first N/O ratio in thefirst silicon titanium oxynitride buffer layer 36 for thermalstabilization of the Mach-Zehnder device. Depending on the resistivitiesrequired, the first and second silicon titanium oxynitride buffer layers36 and 37 are formed with different fixed percentage targets atdifferent Si/Ti ratios. Alternatively, the first silicon titaniumoxynitride buffer layer can be formed with a gradient change in theratio of N/O like the graded silicon titanium oxynitride buffer layerdiscussed with regard to FIG. 6 but with less of a gradient change.

Another alternative is that the first silicon titanium oxynitride bufferlayer 36 on the lithium niobate substrate in FIG. 7 may be doped withindium, or other rare earth metals, in metal or oxide form to furtherimprove temporal stability. This can be accomplished by exposing anothertarget (i.e., multi-source deposition including both a Si₃N₄-TiN targetand a dopant target) within the deposition chamber during the depositionof the first silicon titanium oxynitride buffer layer 36. Another methodwould be to form the first silicon titanium oxynitride buffer layer 36with a single Si₃N₄-TiN target containing the dopant for improvingtemporal stability.

FIG. 8 is a cross-sectional view of a fourth exemplary embodiment of thepresent invention having two buffer layers of silicon titaniumoxynitride for suppressing both thermal and temporal effects inconjunction with a SiO₂ buffer layer that further prevents the opticalsignal from being absorbed by the electrodes in a Mach-Zehnderinterferometer. The Mach-Zehnder interferometer includes a Z-cut lithiumniobate substrate 40 in which optical waveguide paths 44 a and 44 b arediffused with a dopant, for example titanium, to form the opticalwaveguide paths. Electrodes 42 a and 43 are respectively positionedabove or directly above the first optical waveguide paths 44 a and thesecond optical waveguide path 44 b. In addition, another electrode 42 bis positioned on the substrate so that electrodes 42 a and 42 b are oneach side of the electrode 43, which is an input signal electrode. Anundoped SiO₂ buffer layer 45 is formed on the surface of the Z-cutlithium niobate substrate 40 above the optical waveguide paths 44 a and44 b. A first silicon titanium oxynitride buffer layer 46 having a firstSi/Ti ratio and a first N/O ratio is positioned on undoped SiO₂ bufferlayer 45 below electrodes 42 a, 42 b and 43, and above the opticalwaveguide paths 44 a and 44 b. A second silicon titanium oxynitridebuffer layer 47 having a second Si/Ti ratio and a second N/O ratio ispositioned on the first silicon titanium oxynitride buffer layer 46below electrodes 42 a, 42 b and 43, and above the optical waveguidepaths 44 a and 44 b. The thickness of the undoped SiO₂ buffer layer 45is about 0.1-1.5 micron. The thickness of the first silicon titaniumoxynitride buffer layer 46 is about 0.1-1.0 micron and the thickness ofthe second titanium oxynitride buffer layer 47 is about 0.1-1.0 micron.Similarly as previously discussed with regard to FIG. 4, a connectinglayer 48 on the sides of the lithium niobate substrate 40 interconnectsthe second silicon titanium oxynitride buffer layer 47 to the +Z face(bottom surface) of the lithium niobate substrate 40.

The first Si/Ti ratio in the first silicon titanium oxynitride bufferlayer 46 is larger than the second Si/Ti ratio in the second silicontitanium oxynitride buffer layer 47 for temporal stabilization of theMach-Zehnder device. The second N/O ratio in the second silicon titaniumoxynitride buffer layer 47 is larger than the first N/O ratio in thefirst silicon titanium oxynitride buffer layer 46 for thermalstabilization of the Mach-Zehnder device. Depending on the resistivitiesrequired, the first and second silicon titanium oxynitride buffer layers46 and 47 are formed with different fixed percentage targets atdifferent Si/Ti ratios. Alternatively, the first silicon titaniumoxynitride buffer layer can be formed with a gradient change in theratio of N/O like the graded silicon titanium oxynitride buffer layerdiscussed with regard to FIG. 6 but with less of a gradient change. Asdescribed with regard to FIGS. 6 and 7 above, the first silicon titaniumoxynitride buffer layer 46 on the undoped SiO₂ buffer layer 45 may bedoped with indium or other rare earth metals, in metal or oxide form tofurther improve temporal stability.

Although the silicon titanium oxynitride films described above are inZ-cut optical devices, the films may also be used on any orientation ofLiNbO₃. For example, a high Si/Ti ratio and low N/O ratio silicontitanium oxynitride may be used on the top surface of X-cut LiNbO₃devices for temporal stabilization. Also the device illustrated in FIG.2 may be formed such that a X axis of the crystal orientation extends inlongitudinal direction of the lithium niobate substrate 1 along thewaveguide paths 4 a and 4 b, with the Y axis replacing the X axis shownin the figure. In addition, the buffer structures described above can beused in other optical devices formed within LiNbO₃, LiTaO₃ or the likeelectro-optical materials, such as polarizers or optical switches, forthermal and/or temporal stabilization. Furthermore, other elements incolumn 4 (IVB) of the periodic table can be substituted for titanium inthe disclosure above like, for example, Zirconium (Zr). For example, thefirst buffer layer in the device of FIG. 8 may be silicon zirconiumoxynitride while the second buffer layer is silicon titanium nitride.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the optical waveguide deviceof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A method for forming an optical waveguide device comprising the stepsof: forming an optical waveguide path within a surface of theelectro-optical crystal substrate; forming a buffer layer comprisingsilicon, an element in column 4 (IVB) of the periodic table, oxygen, andnitrogen, positioned above the optical waveguide path; forming at leastone electrode positioned above the buffer layer for applying an electricfield to the optical waveguide path; forming a connecting means forinterconnecting the thermal stabilization buffer layer to anothersurface of the electro-optical crystal substrate that is opposite to thesurface in which the optical waveguide path is formed.
 2. The method forforming an optical waveguide device of claim 1, wherein the buffer layeris sputter deposited using a target comprised of silicon nitride and anitride of an element in column 4 (IVB) of the periodic table.
 3. Themethod for forming an optical waveguide device of claim 2, wherein thebuffer layer is sputter deposited in atmosphere containing O₂ and N₂. 4.The method for forming an optical waveguide device of claim 2, whereinthe target further comprises a rare earth metal.
 5. The method forforming an optical waveguide device of claim 2, wherein an additionaltarget containing a rare earth metal is exposed while the buffer layeris sputter deposited.